While the induced reactivation of organophosphate-inhibited acetylcholinesterase (AChE) has been extensively studied, hydrolysis of the inactive adduct by water, i.e., spontaneous reactivation, has received relatively little attention. We have found that the conventional preparations of electric eel AChE contain enzyme species that are resistant to "aging" and reactivate rapidly following exposure to several "irreversible" inhibitors. This proposal outlines a program in which the rapidly reactivating AChE components would be used to study the basis of spontaneous reactivation after treatment with DFP (diisopropylphosphorofluoridate) and other inhibitors. The reactivation kinetics themselves and their perturbation by various catonic ligands would be examined. Initially, preparations of electric eel and human erythrocyte AChE in which the bulk non-reactivating enzyme had been inhibited would be used. Later, the rapidly reactivating component(s) would be purified from these sources and used in studies that compared the subunit heterogeneity and active site geometry of the reactivating and non-reactivating components. While this study will be concerned mostly with elucidating the mechanism of spontaneous reactivation of AChE, acceleration of the rate of spontaneous reactivation could be therapeutically useful.